Circuit board test fixtures are devices which are used in testing electronic circuits, often referred to as "devices under test" (DUTs). The DUT is often a printed circuit board whose constituent components are to be tested. In use, the fixture generally holds and positions the DUT and establishes electrical interconnection between test equipment and test points or nodes on the DUT. The test equipment typically includes signal circuitry for generating selected excitation signals applied to the DUT test points through the fixture interconnections and for detecting or monitoring response signals therefrom as part of a manufacturer's quality assurance testing program or as a diagnostic procedure during circuit board repair and servicing.
One type of test fixture is the so-called "bed of nails" device which, as the name suggests, has an array of spring-loaded contact probes which support and make electrical contact with the DUT. The probes are mounted at fixed locations in an interface plate in registration with all desired test points on the circuit board. The other ends of the probes communicate electrically with the test signal circuitry. During testing, the DUT is disposed in the test fixture, which is typically placed within or equipped with a device for the application of pressure to make the requisite probe-to-DUT contacts.
Although known versions of such test fixtures are generally suitable for their intended purpose, certain inherent limitations have been identified. One such limitation stems from the fact that each different DUT will generally have test points arranged in a pattern unique to that DUT. For testing, the fixture's probes are arranged in the identical grid pattern. When there are many test points on the DUT which must be contacted by as many individual probes of the fixture, the fabrication of the probe fixture is complex and time-consuming. The interface plate has to be designed and manufactured, and all probes individually wired, specifically for the particular DUT. Of course, that probe fixture is then suitable only for testing boards having that particular layout. If any modification is made to the printed circuit board components or circuitry that necessitates a change in the location of its test points, the test fixture must be redesigned and often rebuilt to account for the change in the test point pattern. Then the probes must again be wired to the test circuitry.
In recent years, an additional concern has developed with regard to the design of probe fixtures. As the density of electronic components, particularly integrated circuits (ICs), has increased, the need for smaller and more-closely spaced test points has arisen. For example, it is not uncommon today to find surface-mounted devices (SMDs) which have individual leads spaced apart at a pitch of 25 mils (thousandths of an inch) or less. This has exacerbated the problem of properly fabricating a test fixture so that the relative positioning of the probes matches that of the test points of the DUT. In addition, when applied to the DUT, the fixture must be precisely aligned relative to the DUT to ensure that the probes make contact with the correct test points.